The gastrointestinal microbiome forms a bridge between diet, mammalian metabolism, and immunity. Gut microbes can modulate innate and adaptive immune responses in the gut via different signaling pathways, and suppression of inflammation may alter predisposition to cancer, specifically colorectal neoplasms. Bacterial genes in the microbiome encode for enzymes that perform biochemical conversion of dietary amino acids into various microbial metabolites. We propose that amino acid metabolites derived from the human microbiome suppress neoplasia by inhibiting mucosal inflammation and cell proliferation. Based on recent findings, intestinal bacteria can convert amino acids, such as L-histidine, into biogenic amines, such as histamine, that suppress pro-inflammatory cytokine production by impeding MAP kinase signaling. Several L-histidine metabolites have demonstrated immunomodulatory effects and raise the possibility that luminal conversion by gut microbes may be an important strategy for diet-mediated cancer prevention. The overall hypothesis is that L-histidine metabolites produced by gut microbes, including but not limited to histamine, suppress chronic intestinal inflammation and inflammation-associated colonic neoplasia by regulating Stat3 and MAP kinase signaling pathways. In Aim 1, suppression of acute intestinal inflammation by L-histidine metabolites of the human gut microbiome will be explored. Bacterial strains with potent immunomodulatory and histidine-metabolizing activities have been isolated from the human microbiome. L-histidine pathways and metabolites will be explored by gene expression and metabolomics studies of gut lactobacilli. Endowment of the mouse gut microbiome with histidine-metabolizing, histamine-generating genes from the human microbiome will facilitate studies of the interplay between diet and the intestinal microbiome in vivo. In Aims 2 and 3, two different mouse models (DSS-ApcMin/+ and Hdc-/-) will be used to explore the importance of L-histidine metabolism and histamine generation in the biology of chronic intestinal inflammation and colorectal cancer. Isotopically labeled L-histidine will be deployed as a tracer to explore gut microbiome- mediated amino acid metabolism in vivo. These studies may point the way towards an improved understanding of how diet and the microbiome affect cancer risk in human populations.